Does OSL work on the GPU?
From the wiki:


  Users can now create their own nodes using the Open Shading Language (OSL). Note that these nodes will only work for CPU rendering; there is no support for running OSL code on the GPU.


Quote from Brecht:


  The main issue is to get OSL working on the GPU. It's almost as much work as building Cycles itself. It's not easy to do so we need more people to help us.


The oso file just contains the shader compiled bytecode.